Paper Broker override CalculateCommission

I am trying to implement an override method for CalculateCommission, but am getting errors with regard to references. When I build in .NET 4.5, there is no method CalculateCommissions to override. Should I use a different .NET Framework such as 3.5? All the rest I have built on 4.5 with no problem.

Error1'IGCFDBroker.IGCFDBroker.CalculateCommission(RightEdge.Common.BrokerOrder,double)': no suitable method found to overridec:\users\jstumpf\documents\visualstudio 2013\Projects\IGCFDPaperBroker\Class1.cs3735IGCFDPaperBroker